JPH03131230A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE:To shorten a restriction time of a patient and to attempt to improve through-put by constituting a sequencer controlling pulse sequences of each specified exciting pulse, an encode inclined magnetic field and a pulse and providing a function obtaining different images by one photographing. CONSTITUTION:A pulse sequence having three pulses consisting of the 1st pulse wherein a user can arbitrary select the angle, the 2nd 90 deg. pulse and the 3rd 180 deg. pulse and a signal of a transvere spin component excited by means of the 1st pulse is formed in a positive and negative frequency encode inclined magnetic field. Then, a transverse magnetization component is dispersed by means of an inclined magnetic field called a spoiller and the remaining longitudinal magnetization component is formed by means of the 2nd and 3rd pulses. Therefore, two kinds of images, for example, a longitudinal relaxation emphasizing image and a transeverse relaxation emphasizing image can be obtd. by one photographing. It is thereby possible to shorten a restrictive time of an examined body and to improve through-put.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention provides a nuclear magnetic resonance imaging apparatus that utilizes nuclear magnetic resonance (NMR) phenomena to obtain tomographic images of a subject. The present invention relates to an MRI apparatus characterized in that an information image is obtained in one imaging session.

[Conventional technology]

Nuclear magnetic resonance imaging equipment utilizes nuclear magnetic resonance phenomena to determine the nuclear spin (
Hereinafter, this will be simply referred to as spin. ), the density distribution, relaxation time distribution, etc. are measured, and the cross section of the object is displayed as an image from the measured data.

In this apparatus, a subject 1 is placed in a static magnetic field generator 10 (not shown in FIG. 1) which generates a static magnetic field of about 0.02 to 2 Tesla. At this time, the spins in the subject perform precession with the direction of the static magnetic field as an axis at a frequency determined by the strength Ho of the static magnetic field. This frequency is called the Larmor frequency, and the Larmor frequency ν0 is γ yao = -Ho...(1)
It can be expressed as 2π. Here, γ is the gyromagnetic ratio and has a unique value for each type of atomic nucleus. Further, if the angular velocity of Larmor precession is ω0, the relationship ωa”2πνO (2) is given, so it is given by ω0=γHo (3).

Here, when a high frequency magnetic field (1! magnetic wave) of a frequency equal to the Larmor frequency 90 of the atomic nucleus to be measured is applied by the high frequency irradiation coil 20a, the spins are excited and transition to a high energy state.

When this high-frequency magnetic field is cut off, the spins return to their original low energy state. The electromagnetic waves emitted at this time are received by the high-frequency receiving coil 20b, amplified and waveform-shaped by the amplifier 23, and then sent to the A/D converter 25 (hereinafter referred to as ADC).

) and send it to the central processing unit 11 (hereinafter referred to as CPU). The CPU 11 performs reconstruction calculations based on this data, and the calculated data is displayed on the display 28 as a tomographic image of the subject 1. The above-mentioned high frequency magnetic field is obtained by amplifying a signal sent out by the sequencer 12 controlled by the CPU II by the high frequency transmitting coil amplifier 19 and sending the amplified signal to the high frequency transmitting coil 20a.

In addition to one or more static magnetic fields 10 and a high-frequency magnetic field, the MRI apparatus includes a gradient magnetic field coil group 24 to generate gradient magnetic fields for obtaining position information in space. These gradient magnetic field coils 24 are supplied with current from a gradient magnetic field coil power supply 25 operated by signals from the sequencer 12, and generate gradient magnetic fields.

Here, the imaging principle of the MHI device will be described.

As shown in Fig. 4(a), an atomic nucleus placed in a static magnetic field Ho in the Z direction behaves like a single bar magnet when viewed from classical physics, and the Z It precesses around its axis.

This frequency is given by the above equation (2) and is proportional to the strength of the static magnetic field. γ in equations (1) and (3) is called the gyromagnetic ratio, and has a value specific to the fJK nucleus.

In general, there are a huge number of atomic nuclei to be measured, and each one rotates with an arbitrary phase, so when viewed as a whole,
- The components in the Y plane cancel each other out, and macroscopic magnetization remains only in the Z direction component. In this state, a high frequency magnetic field H1 with a frequency equal to the Larmor frequency ν0 is applied in the X direction (Fig. 4(b)
)), the macroscopic magnetization begins to fall in the Y plane. This tilting angle is approximately proportional to the product of the amplitude of Hl and the application time, and Hl when tilting 90 degrees with respect to the pulse application time is called a 90'' pulse, and Hl when tilting 180' is called a 1801 pulse.

Now, a two-dimensional Fourier imaging method is currently commonly used for imaging using an MHI device. A typical pulse sequence of the spin echo method, which is a typical method among these methods, is shown in FIG.

In this pulse sequence, first, 900 pulses 29 are applied, and then a 180' pulse 30 is applied after a time of Te/2, where Te is the echo time.

After applying the 90'' pulse 29, each spin starts rotating in the x-y plane at its own unique speed, so that a phase difference occurs between the spins over time.

When a 180° pulse 30 is added here, each spin becomes the 6th
As shown in the figure, the spins are symmetrically reversed around the
form an echo signal.

Although the signals are measured as described above, the spatial distribution of the signals must be determined in order to construct a tomographic image. For this purpose, a linear gradient magnetic field is used.

A spatial magnetic field gradient is created by superimposing a gradient magnetic field on a uniform static magnetic field. As mentioned earlier, the rotational frequency of spins is proportional to the magnetic field strength, so when a gradient magnetic field is applied, the rotational frequency of each spin differs spatially6. Therefore, by examining this frequency, each spin It is possible to know the location of For this purpose 1. :, phase encoding gradient magnetic field 9 frequency encoding gradient magnetic field is used.

Using the pulse sequence described above as a basic unit, the pulse sequence is repeated a predetermined number of times, for example, 256 times, at a constant repetition time (TR) while changing the strength of the phase encode gradient magnetic field each time.
The spatial distribution of macroscopic magnetization can be determined by subjecting the thus obtained measurement signal to two-dimensional inverse Fourier transformation. In the above explanation, the three types of gradient magnetic fields must not overlap with each other.

It may be any one of x, y, and z, or a combination thereof.

Regarding the basic principles of MRI, please refer to ``rNMR Medicine'' (
Basic and Clinical) (edited by the Nuclear Magnetic Resonance Medical Study Group, Kujun Co., Ltd., published January 20, 1980).

By the way, among the principles mentioned in ``2'', regarding the relaxation phenomenon,
I will explain in more detail. Relaxation is when spins excited by a high-frequency magnetic field release absorbed energy,
It means returning to the ground state with a certain time constant. There are two types of this relaxation phenomenon, one called longitudinal relaxation and the other transverse relaxation.Longitudinal relaxation is also called spin-lattice relaxation, and the time constant of relaxation, that is, longitudinal relaxation time, is expressed as T1.

Transverse relaxation is also called spin-spin relaxation, and the transverse relaxation time is expressed as T2.

Longitudinal relaxation is a process in which the Z'-axis component M z ' of macroscopic magnetization M returns to be equal to the magnitude Mo of M in the steady state, and M7. ' increases exponentially as shown in FIG.

The longitudinal relaxation time T1 is the time until Mz'=Mo(], -1). 1'1 indicates the speed at which the spin releases the absorbed energy into the surrounding lattice as thermal vibration energy and returns to a steady state, which varies depending on the molecular bonding state of the sample.

That is, even for the same hydrogen nucleus (proton), water and fat have different T1 values, and in the human body, there are tissue-specific T1 values. Figure 9 shows this.

Transverse relaxation is a process in which each spin is equally defeated around the 2' axis, and the vector sum MX'Y'' on the X'-Y' plane decays exponentially (see Figure 8) and returns to 0. The transverse relaxation time T2 is the time until MX'Y'=Mo/e.Similar to T1, T2 also varies depending on the molecular bonding state of the sample.Figure 10 shows the difference in TZ value depending on the tissue. Shown below.

As mentioned above, T I T T 2 is a tissue-specific value and can also identify lesions such as tumors, so imaging this two-dimensional distribution will contribute to improving diagnostic performance. can. The T1-weighted image, 12
This is an enhanced image, with the inversion time TR, echo time TE, and inversion recovery (I).
This can be obtained by appropriately selecting the recovery time TI in the R) method and the flip angle in the gradient echo method (hereinafter referred to as the -GE method). The signal strength S of each tissue in the spin echo (SE) method shown in Fig. 5 is: 5=N(H)(1-2exp((TR=TE/2))e
xp(-TR/Ti))exp(TE/T2)...(
4) is given by. Here, N(H) indicates the proton density 6 Normally, TR>TE/2, so S=:N(H
)(], -exp(-TR/Ti))exp(-TE/
T2)...(5) can be approximated as follows.

From equation (5), when T R> T l, exp(
-T R/ T I) ~ O- (6), and the influence of T1 can be ignored.7 On the other hand, when TE<TZ, QXP (-T E / T 2) ~1
...(7), and the influence of T2 can be ignored.

From equations (5) and (6), the long i TR, long TE SE method (
Hereinafter, it will be referred to as long-SE. ),
5=N(H)exp(-TE/Tp,)”
48), and a 12-enhanced image is obtained. On the contrary, short TR
.

When using the short TE SE method (hereinafter referred to as 5hort-SE), from equations (5) and (7).

S = N (H)(], -exp(-TR/T
i)) (9), and a T1-weighted image is obtained.

The IR method shown in FIG. 11 will be explained. The signal S by the IR method is 5=N(H)(1-2exp(-TI/Ti)+2ex
p(-(TR-TE/2)/Tz)-exp(-TR
/T t))exp(-TE/'rz)...
(10) is given by. Here, if TR>TE/2, then 5=
N(H)(1-2exp(-TI/TI)+exp(-
T R / T z)) exp (-T E / Tx)・
...(11) becomes. Furthermore, if TE<T2, 5=N(H)(
1-2exp(-TI/Tx)+exp(TR/T
l)) - (12), and a T1 weighted image is obtained.

The GE method shown in FIG. 12 will be explained.

In the GE method, the phase of the spins is aligned by reversing the frequency encoded gradient magnetic field without using a 180' pulse. Therefore, the component that relaxes due to the unevenness of the static magnetic field depending on the location does not converge. The transverse relaxation time taking uniformity into consideration is expressed as Tx umbrella. In addition, selective excitation pulse 3
1, that is, the angle of macroscopic magnetization that is flipped by the selective excitation pulse 31, is called a flip angle. In other words.

A pulse with a flip angle θ0 means that the macroscopic magnetization is 00
It is a pulse that defeats.

The signal S obtained by the GE method in FIG. 12 is S = N
(H)exp(-TE/Tz fist) [(1-IBXP(-
TR/T1)) sinθko/Tl-exp(TR/Tl
)axp(-TR/T2)-(exp(-TR,/Tl
)-exp(-TR/Tz))cosθco...(13
) is given by. Here, θ indicates the flip angle,
When TR>Tz, 5=N(H)exp(-TE/T2[(1-exp(
-TR/Tt)) sinθ]/[:1-exp(-TR
/Tt)cosθco-(14). θ~O”, TR>T1, then 5=N(H)
exp(TE/Tz*) −(15),
A T2 weighted image is obtained.

As described above, enhanced images can be obtained by various methods. For information on the operating principles of the IR method and the GE method, and their enhanced images, please refer to Magnetic Resonance Imaging.

(1988) pp. 66-83 (Magneti
cResonance Imaging (1988)
For details, see pp. 66-83). Also, regarding the relaxation phenomenon mentioned above.

Application of rNMR to biological systems” (Nishimura Shoten, 1985)
In addition, "rNMR Diagnostic Methods - From Basics to Clinical Practice", written by Isamu Mano, published by Shujunsha on April 1, 1984, pp. 34 to 3.
Details on page 7.

[Problem to be solved by the invention]

As mentioned above, conventionally, long-SE emphasizes T2,
Alternatively, T211 emphasis with GE method, IR method or 5h
It was necessary to perform T1 weighting at ort-S E and to perform imaging two or more times in separate sequences. In this case, imaging takes a long time and the subject, ie, the patient, is restrained for a long time, which not only causes pain but also causes deterioration in image quality such as artifacts due to single body movements. Moreover, this was an obstacle to improving throughput.

An object of the present invention is to solve this problem and provide a pulse sequence that makes it possible to obtain two different types of images, for example, Tt and Tz*-enhanced images, in one imaging operation.

[Means to solve the problem]

In order to achieve the above purpose, the first pulse, the second 90" pulse, and the third 18" pulse, the angle of which can be selected arbitrarily by the user, are used.
A pulse sequence with three 0° pulses is provided, and the transverse magnetization component of the spins excited by the first pulse is formed into a signal by a gradient magnetic field that encodes positive and negative frequencies, and then by a gradient magnetic field called a spoiler. The transverse magnetization component is diffused, and the remaining longitudinal magnetization component is formed by the second and third pulses.

[Effect]

According to the present invention, the first angle can be arbitrarily selected by the user.
A pulse sequence having three pulses, a second 90' pulse, and a third 180° pulse is provided;
The transverse magnetization component of the spins excited by the pulse is formed into a signal using a gradient magnetic field that encodes positive and negative frequencies, and then,
The transverse magnetization component is diffused by a gradient magnetic field called a spoiler, and the remaining longitudinal magnetization component is formed by the second and third pulses. Therefore, two types of images, for example, a T1-weighted image and a T2-weighted image, can be obtained in one imaging operation, so that clinically useful information can be provided in a short time.

〔Example〕

The following five embodiments of the present invention are shown in Fig. 1, Fig. 2, and Fig. 1.
This will be explained with reference to Figure 3. FIG. 1 is a block diagram illustrating the overall configuration of a nuclear magnetic resonance imaging apparatus to which the present invention is applied, and FIG. 2 is a schematic diagram illustrating a pulse sequence according to an embodiment of the present invention.

A nuclear magnetic resonance imaging apparatus to which the present invention is applied will be explained with reference to FIG. This nuclear magnetic resonance imaging device
Roughly speaking, it comprises a central processing bag [(CPU) 11, a sequencer 12, a transmission system 13, a static magnetic field generating magnet 10, a reception system] 5, and a signal processing system 16.

The central processing unit (CPU) 11 executes a sequencer according to a predetermined program]2. Transmission system] 3. It controls each of the receiving system 1s and the ff1 processing system 16. The sequencer J, 2 receives #@I from the central processing unit 11.
A system that operates based on commands and transmits various commands necessary for data collection of tomographic images of subject 1]3. Static magnetic field generating magnet], 0 gradient magnetic field generating system 14. The signal is sent to the receiving system 15.

The transmission system 13 includes a high frequency oscillator 17, a modulator 18, and an irradiation coil 20a as a high frequency coil.

A modulator 18 modulates the amplitude of the high frequency pulse from the high frequency oscillator 1.7 according to a command from the sequencer 12, and the amplitude modulated high frequency pulse is amplified via the high frequency amplifier 19 and supplied to the irradiation coil 20a, thereby producing a predetermined signal. The subject 1 is irradiated with pulsed electromagnetic waves.

The static magnetic field generating magnet 1.0 is for generating a uniform static magnetic field around the subject J in any direction. Inside this static magnetic field generating magnet, in addition to the irradiation coil 20a, a gradient magnetic field coil 21 for generating a gradient magnetic field and a receiving coil 201 of the receiving system 15 are installed. Gradient magnetic field generation system]-4 includes a gradient magnetic field coil 21 having a configuration capable of independently applying gradient magnetic fields in the directions of mutually orthogonal Cartesian coordinate axes, a gradient magnetic field power source 22 that supplies current to the gradient magnetic field coils, and a gradient magnetic field power source 22. It is configured by a sequencer 12 that controls it.

The receiving system 15 includes a receiving filter 20b as a high frequency coil.
and an amplifier 23, a quadrature phase detector 24, and an A/D converter 25 connected to the receiving coil 20b, and when the receiving coil 20b detects an NMR signal from the subject 1, the signal is transmitted to the amplifier 23°. Quadrature phase detector 24. It is converted into a digital quantity via the A/D converter 25, and is also converted to a digital quantity by the quadrature phase detector 24 at the timing according to the command from the sequencer 12.
The collected data is converted into two series of sampled data and sent to the central processing unit], 1°.

The signal processing system 16 includes a magnetic disk 26. optical disk 27
A display 2 consisting of an external storage device such as, and a CRT etc.
8, and the data from the receiving system 15 is sent to the central processing bag [1
1, the central processing unit 11 executes processing such as signal processing 2 and image reconstruction, and displays the resulting desired cross-sectional image of the subject on the display 28, and also displays data in the external storage device. The information is recorded on a magnetic disk 26 or the like.

22-5 A pulse sequence of an embodiment of the present invention will be explained with reference to FIG. In FIG. 2, the first pulse 32 is an excitation pulse whose angle can be selected arbitrarily by the user.

Here, let that angle be ao. When the first pulse is applied, the macroscopic magnetization is inverted by α6 (Fig. 13), where the inverted macroscopic magnetization is the vector sum of the Z-axis component Mz and the xy plane component M x y. The expression a M z is called the longitudinal component of macroscopic magnetization, and M x y is called the transverse component.

First, the description will focus on the vertical component. In the 9 sections ■-■ where the longitudinal component causes a longitudinal relaxation phenomenon with a time constant Th of 1,
The longitudinal component does not feel one phase and frequency encoding gradient field. Therefore, no signal is generated in one section (3), after which the vertical component is collapsed into the X7 plane by the 90' pulse 33. Then, the frequency enrichment and phase encoding are felt (section x), and the 180" pulse 34 causes the diffused spins to return, and a signal can be obtained in the section XrV. In this way, the vertical component and the If we focus on the magnetization, it can be thought of as the XR method.

Next, explanation will be given focusing on magnetization which becomes a lateral component due to the α° pulse. The magnetization, which has become transverse, feels phase encoding in section ■, and also feels a frequency encoding gradient magnetic field in the negative direction. Further, the frequency encoding gradient magnetic field is reversed, and the spins sense a positive gradient magnetic field (section V) and converge, and a signal can be obtained in section (2). Furthermore, in section ■, due to a large gradient magnetic field for diffusion called a spoiler, the transverse magnetization is forced to undergo transverse relaxation, and the transverse component becomes 0.
become. After that, since the horizontal component is 0, no signal is formed in the interval layer. In this way, if we focus on the magnetization that becomes a lateral component due to the α° pulse, it can be considered like the GE method. As a result, in section (2), the magnetization that becomes a horizontal component due to the α° pulse forms a signal using the GE method, and in the section layer, the magnetization that becomes a longitudinal component due to the α pulse forms a signal using the IR method. Here, Tel, Te2, T
I.

By arbitrarily setting α and repetition time TR,
A variety of two different images can be obtained.

〔Effect of the invention〕

According to the present invention, since two different types of images can be obtained in one imaging operation, there is an effect that a large amount of diagnostically useful information can be obtained in a short time.

Furthermore, since a large amount of information can be obtained with one imaging, the time for restraining the subject can be shortened, which has the effect of improving throughput, and further has the effect of suppressing artifacts due to body movement.

[Brief explanation of the drawing]

FIG. 1 is a block explanatory diagram of the overall configuration of a nuclear magnetic resonance imaging apparatus to which the present invention is applied, FIG. 2 is a schematic explanatory diagram of the pulse sequence of the present invention, and FIG. 3 is an MRI apparatus to which the present invention is applied. A block diagram, Fig. 4 is an explanatory diagram of macroscopic magnetization, Fig. 5 is a schematic explanatory diagram of the pulse sequence of the SE method, and Fig. 6 is an explanatory diagram showing the behavior of magnetization due to the application of a 180' pulse in the SE method. Fig. 7 is an explanatory diagram showing the longitudinal relaxation phenomenon, Fig. 8 is an explanatory diagram showing the transverse relaxation phenomenon, Fig. 9 is an explanatory diagram showing the longitudinal relaxation in the human body, and Fig. 10 is an explanatory diagram showing the transverse relaxation in the human body. 11 is a schematic explanatory diagram of the pulse sequence of the IR method, FIG. 12 is a schematic explanatory diagram of the pulse sequence of the GE method, and FIG. 13 is the diagram after application of the first excitation pulse in the embodiment of the present invention. It is an explanatory diagram of magnetization of. 1... Subject, 11... Central processing unit (CPU)
, 12... Sequencer, 32... α0 pulse, 33
...90° pulse, 34...180@pulse. Chime-Kayaotsu Captive Stem 4 (Ku) Cb) Chiga 5 Echo No. 411 Giwa-mecha 1゜me-Kaya 2 Honey f Nii-ni) rε ME Potato/3

Claims (1)

[Scope of Claims] 1. Means for applying a static magnetic field to a subject; a means for applying a slicing direction gradient magnetic field, a frequency encoding gradient magnetic field, and a phase encoding gradient magnetic field to the subject; means for repeatedly applying a high frequency pulse that causes magnetic resonance in a certain predetermined pulse sequence;
A nuclear magnetic resonance imaging apparatus comprising a means for detecting a nuclear magnetic resonance signal, wherein the sequencer for controlling the pulse sequence is configured such that a user can set an arbitrary angle.
Consists of two excitation pulses, a positive and negative frequency-encoded gradient magnetic field that images the signal of the transverse magnetization component excited by the pulse, and one 90° pulse and one 180° pulse that images the signal of the longitudinal magnetization component. A magnetic resonance imaging apparatus characterized in that it has a function of obtaining images with two different types of information in one imaging. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the two types of images are a longitudinal relaxation-enhanced image and a transverse relaxation-enhanced image.